The dress had an unflattering cut that made her look heavier than she was.

unflattering 是由前缀 un-（表示否定）和形容词 flattering（讨人喜欢的）组成的复合词。flattering 来自动词 flatter（奉承、使显得好看），所以 unflattering 意思是"不讨人喜欢的、不显身材的"，指衣服的款式或剪裁让人看起来不如实际好看。